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Silicon has 23 isotopes with mass numbers ranging from 22 to 44. Remember, isotopes of element have the same number of protons, but different amounts of neutrons. The mass number tells how many protons and neutrons an isotope has. There are three naturally occurring isotopes of silicon and twenty isotopes that are artificial.
|Atomic mass||28.085 atomic mass units|
|Molar mass/molecular weight||28.085 g/mol|
|Luster||Blue-gray metallic sheen|
|Melting point/freezing point||1414C, 2577F|
|Boiling point||3265C, 5909F|
|Density||2.3296 g cm -3|
|State of matter at room temperature (normal phase)||Solid|
|Hardness (Vickers scale)||9630.1303 MPa|
|Electrical conductivity||1000 S/m|
|Thermal Conductivity||150 W/(m K)|
|Dielectric constant/relative permittivity||11.7|
|Specific heat capacity||0.7 J g -1o C -1|
|Resistivity||6.4 x 10 2 ohm-m|
|Youngs modulus||140-180 GPa|
|Tensile strength||165-180 MPa|
|Oxidation states/ionic charge||+4, +2, -4|
Silica has been used by human civilization for a long time. Some tools made by first humans had silica flints. In fact, name is derived from the Latin word for 'flint', silicis. Chemists weren't really interested in the structure of silica until Antoine Lavoisier proposed that it was an element in 1787. In the early 1800s, another chemist, Sir Humphry Davy, who was famous at the time for finding elements, proclaimed that silica was in fact a compound, made up of more than one element. However, his tried and true method of element discovery using electrolysis didn't yield elements from silica. Swedish chemist Jons Jacob Berzelius gets credit for discovering Silicon. He isolated elements by heating potassium metal with potassium fluorosilicate in 1824.
In order to meet requirements of normal valence, it is sometimes necessary to have more than one bond, that is, more than one shared pair of electrons between two atoms. A case in point is formaldehyde, CH 2 O. In order to provide Carbon with four bonds in this molecule, we must consider Carbon as forming two bonds to oxygen as well as one to each of two hydrogens. At the same time, the oxygen atom is also provided with two bonds its normal valence require: note that all four share electrons in the Carbon-oxygen bond are included both in the octet of Carbon and in the octet of oxygen. A bond involving two electron pairs is called a double bond. Occasionally, usual valences of atoms in molecules do not tell us what skeleton structure should be. For example, in carbon monoxide, CO, it is hard to see how one carbon atom can be matched with a single oxygen atom. In cases like this, where valences appear to be incompatible, counting valence electrons usually leads to a structure which satisfies the octet rule. A carbon has 4 valence electrons and oxygen has 6, for a total of 10. We want to arrange these 10 electrons into two octets, but two separate groups of 8 electrons would require 16 electrons. Only by sharing 16-10, or 6, electrons can we satisfy the octet rule. This leads to a structure where three pairs of electrons are shared between two atoms, and we have a triple bond. Double and triple bonds are not merely devices for helping to fit Lewis diagrams into octet theory. They have objective existence, and their presence in molecules often has a profound effect on how they react with other molecules. Triple bonds are invariably shorter than double bonds, which in turn are shorter than single bonds. For instance, carbon-oxygen distance is 114 pm, in it is 121 pm, while in both ethyl alcohol and dimethyl ether and methanol it is 142 pm. Below are 3-D Jmol images of carbon monoxide, formaldehyde, and methanol, to compare the difference in bond length with. This agrees with the wave-mechanical picture of chemical bonds as being caused by concentration of electron density between nuclei. The more pairs of electrons which are share, greater this density and the more closely atoms are pulled together. In line with this, we would also expect multiple bonds to be stronger than single bonds. Indeed, bond energy of CO is found experimentally to be 360 kJ mol-1, while that of is 736 kJ mol-1, and that of is gigantic 1072 kJ mol-1. The triple bond in Carbon monoxide turns out to be the strongest known covalent bond. The formation of double and triple bonds is not as widespread among atoms of periodic table as one might expect.
We also use Lewis symbols to indicate the formation of covalent bonds, which are shown in Lewis structures, drawings that describe bonding in molecules and polyatomic ions. For example, when two chlorine atoms form chlorine molecule, they share one pair of electrons: Lewis structure indicates that each Cl atom has three pairs OF electrons that are not used in bonding and one share pair of electrons. Dash is sometimes used to indicate shared pair of electrons: single shared pair of electrons is called single bond. Each Cl atom interacts with eight valence electrons: six in lone pairs and two in single bond.
Other halogen molecules form bonds like those in chlorine molecule: one single bond between atoms and three lone pairs of electrons per atom. This allows each halogen atom to have a noble gas electron configuration. The tendency of main group atoms to form enough bonds to obtain eight valence electrons is known as the octet rule. The number of bonds that atom can form can often be predicted from the number of electrons needed to reach octet; this is especially true of nonmetals of second period of the periodic table. For example, each atom of group 14 element has four electrons in its outermost shell and therefore requires four more electrons to reach the octet. These four electrons can be gained by forming four covalent bonds, as illustrated here for carbon in CCl 4 and silicon in SiH 4. Because hydrogen only needs two electrons to fill its valence shell, it is an exception to the octet rule. Transition elements and inner transition elements also do not follow the octet rule: group 15 elements such as nitrogen have five valence electrons in atomic Lewis symbol: one lone pair and three unpaired electrons. To obtain octet, these atoms form three covalent bonds, as in NH 3. Oxygen and other atoms in group 16 obtain octets by forming two covalent bonds:
|Group||14 ( Carbon Family )|
|Electron Configuration||3s 2 3p 2|
|Atomic Weight||28.0855 g|
|Melting Point||1414 o C|
|Boiling Point||3265 o C|
|Oxidation States||4, 3, 2, 1, -1, -2, -3, -4|
|Stable Isotopes||28 Si 29 Si 30 Si|
Silicon is a crystalline semi-metal or metalloid. One of its forms is shiny, grey and very brittle. It is group 14 element in the same periodic group as Carbon, but chemically behaves distinctly from all of its group counterparts. Silicon shares the bonding versatility of Carbon, with its four valence electrons, but is otherwise a relatively inert element. However, under special conditions, Silicon can be made to be a good deal more reactive. Silicon exhibits metalloid properties, is able to expand its valence shell, and is able to be transformed into a semiconductor; distinguishing it from its periodic group members.
Silica or Silicon dioxide is a very prominent molecule that has atom Silicon in it. Silica was used to make flints in ancient cultures, and the name Silicon comes from the Latin word for flint. Antoine Lavoisier proposed that silica was an element, but Sir Humphry Davy believed it was compound. Jons Jacob Berzelius prove Davy correct by isolating elements in 1824. Silicon is in group 14 and period 3 of the periodic table and has four valence electrons in its Lewis structure. Four valence electrons mean that Silicon can bond in a way similar to Carbon. Because of this, scientists and science fiction writers have pondered that extraterrestrial life may be Silicon-base. Silicon has atomic number 14, which means it has 14 protons in its nucleus. It has the chemical symbol Si and is classified as a metalloid. It will react with bases and hydrofluoric acid, but no other acids. There are three natural isotopes of Silicon and 20 artificial isotopes of Silicon with mass numbers ranging from 22 to 44.
We also use Lewis symbols to indicate the formation of covalent bonds, which are shown in Lewis structures, drawings that describe bonding in molecules and polyatomic ions. For example, when two chlorine atoms form chlorine molecule, they share one pair of electrons: Lewis structure indicates that each atom has three pairs of electrons that are not used in bonding and one share pair of electrons. Dash is sometimes used to indicate shared pair of electrons: single shared pair of electrons is called single bond. Each atom interacts with eight valence electrons: six in lone pairs and two in single bond.
In almost all cases, chemical bonds are formed by interactions of valence electrons in atoms. To facilitate our understanding of how valence electrons interact, simple way of representing those valence electrons would be useful. The Lewis electron dot diagram is a representation of valence electrons of an atom that uses dots around the symbol of element. The number of dots equals the number of valence electrons in an atom. These dots are arranged to right and left and above and below the symbol, with no more than two dots on side. For example, Lewis electron dot diagram for calcium is simply Figure 1 shows Lewis symbols for elements of the third period of the periodic table.
Writing Lewis Structures NASAs Cassini-Huygens mission detected a large cloud of toxic hydrogen cyanide on Titan, one of Saturn's moons. Titan also contains ethane, acetylene, and ammonia. What are the Lewis structures of these molecules? Calculate the number of valence electrons.: + = 10: + = 14: + = 10: + = 8 Draw skeleton and connect atoms with single bonds. Remember that H is never a central atom: Where needed distribute electrons to terminal atoms: six electrons place on: no electrons remain: no terminal atoms capable of accepting electrons: no terminal atoms capable of accepting electrons Where needed place remaining electrons on the central atom: no electrons remain: no electrons remain: four electrons place on carbon: two electrons place on nitrogen Where need, rearrange electrons to form multiple bonds in order to obtain octet on each atom: form two more C-N bonds: all atoms have correct number of electrons: form triple bond between two carbon atoms: all atoms have correct number of electrons check Your Learning Both carbon monoxide, and carbon dioxide, are products of combustion of fossil fuels. Both of these gases also cause problems: are toxic and have been implicated in global climate change. What are the Lewis structures of these two molecules?
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